1. Field of the Invention
The present invention generally relates to blade type shutters for photographic cameras, and more particularly to a blade type shutter with a mechanism capable of stabilizing the width of an exposure slit formed between a front curtain and a rear curtain while they are travelling across an exposure aperture.
2. Description of the Prior Art
In general, blade type shutters have heretofore been provided with force generating means such as springs for generating a force that will act to open and close the shutter, shutter driving members to which the force generating means are directly secured, pins projecting from the shutter driving members and arms having holes or slots. The pins are fitted in the holes or slots so that the arms may be interlockingly rotated. Similarly, in the conventional arrangement, each blade is pivotably connected to the corresponding arms through the engagement between the pins and the holes or slots.
However, the above-described pin-slot (or pin-hole) connection needs a small gap to be left therebetween, and this gap is prone to result in undesirable looseness during the shutter operation at the points where the pins and slots or holes are connected. This looseness further results in unwanted variations in the positional relationship between the shutter blades. Therefore, such a prior-art blade type shutter involves difficulties in that each time the shutter is cocked, the stationary positions of the shutter blades are varied, thus making it difficult to fix the starting lines of the blades during the shutter operation. In particular, this problem leads to fluctuations in shutter speeds due to changes in the manner in which the camera body is held. Moreover, as the number of shutter operations is increased, the pins and the holes or slots gradually become worn, and this accentuates the tendency for fluctuation in shutter speeds.
U.S. Pat. No. 4,231,650 proposes an improved mechanism for solving the above-described problems. According to this prior proposal, the shutter blade mechanism includes first biasing spring means for urging a front curtain assembly with respect to a base plate and second biasing spring means for urging a rear curtain assembly with respect to the base plate, and each arm is biased in one direction only by the associated springs so that each pin may be consistently kept in contact with the corresponding hole or slot. Thus the influence exerted by the looseness between the pins and the holes or slots is reduced.
However, since this prior arrangement needs two biasing springs for each of the front and rear curtain assemblies, variation between the forces generated by the two springs of each pair has an undesirable influence on the shutter travelling characteristics. In particular, during high-speed shutter operation wherein the exposure slit may be minimized in width, there is a risk of causing the front and rear curtains to travel in a closed state, that is, without forming any slit.